In recent magnetic disk drives, a disk is rotated at high speed, and a head gimbal assembly is driven at high speed to meet a need for large capacity and high recording density. Therefore, air disturbance (wind turbulence) may occur at a relatively high possibility, causing vibration in disk or head gimbal assembly. The wind turbulence vibration may significantly obstruct positioning of a head on a track on a disk being subjected to high-density recording. Since wind turbulence is caused by air disturbance, it randomly occurs, therefore magnitude or a period of wind turbulence is hardly predicted, and consequently when prompt and accurate positioning control of a head is attempted, the control may be complicated and difficult. Furthermore, the wind turbulence vibration may be a sound source of noises, that is, may be a factor of reducing quietness of a drive.
As another problem associated with high-speed rotation due to an effect of air in a drive, power consumption is increased. When a disk is rotated at high speed, air near the disk is drawn thereby and rotated together. On the other hand, since air away from the disk stays still, shear force is generated between the air and the disk, leading to a load of stopping the disk rotation. This is called windage loss, and the windage loss increases with increase in rotation speed. Motor output needs to be large to enable high speed rotation against the windage loss, which necessarily requires large power.
Here, noticing a fact that each of the wind turbulence and windage loss is proportional to density of gas within a drive, the following idea is previously considered: a low-density gas is enclosed in a sealed magnetic disk drive in place of air, so that the wind turbulence and windage loss are reduced.
As the low-density gas, hydrogen, nitrogen, helium and the like are considered. In the light of actual use, helium is considered to be optimal since it may exhibit a large effect, and may be stable and highly safe. A magnetic disk drive having helium gas being enclosed therein solves the above problem, and makes it possible to achieve prompt and accurate positioning control, power saving, and high quietness. In the case that power saving is not considered, it further makes it possible to achieve faster disk rotation or faster driving of a head gimbal assembly, consequently drive performance can be improved.
However, since helium has an extremely small molecular size, and has a large diffusion coefficient, a housing used for a typical magnetic disk drive inevitably has a problem that since sealing ability is bad, helium easily leaks out, resulting in difficulty in keeping drive performance. Thus, for example, a conventional example as described in U.S. Patent Publication No. 2005/0068666 (“patent document 1 ”) is proposed to making it possible to enclose helium, which is leaky, for a long period.
FIG. 14 shows an example of a section diagram of the sealed magnetic disk drive as above. Here, as a region where helium may leak from a housing at a high possibility, a joined region between a base 200, on which a device component 210 is mounted, and a cover 220 is given. To perfectly seal the relevant region, at the joining position 240, an upper part of a side wall of the base 200 and the cover 220 are welded by laser or joined by soldering to each other.
When laser welding or solder joining is performed, material of each of the base 200 and the cover 220 needs to be selected from a viewpoint of durability or reliability and cost. For example, a base molded by aluminum die casting and an aluminum cover formed by pressing or cutting may be selected. Alternatively, a base formed of an aluminum alloy containing a relatively small amount of copper and magnesium by means of cold forging, and an aluminum cover formed by pressing or cutting may be selected.
Furthermore, as a region where helium may leak from a housing at a high possibility, a small opening in a base bottom is given, which is formed for passing electric wires connecting between a flexible printed circuit (FPC) assembly in the housing and a circuit board outside the housing. To perfectly seal that opening while establishing electric wiring, a feed-through 250 having a plurality of pins 260 as shown in FIG. 14 is used, and wiring lines at an FPC assembly side are connected to pins within the housing, and wiring lines at a circuit board side are connected to pins outside the housing.
FIGS. 15 and 16 show a side diagram and a top diagram of the feed-through 250 respectively. A flange 252 of the feed-through 250 is joined by soldering 300 to a stepped portion of the opening in the bottom of the base 200 at a joining position 270 with the base 200. A plurality of steel pins 260 are provided in the flange 252 in a manner of extending in a perpendicular direction to a flange 252 plane. In such a configuration, a sealing material 280 such as glass or ceramic is filled in a space between the flange 252 and each of the steel pins 260 so as to enclose the periphery of each steel pin 260. A material of the flange 252 is selected to be fitted with the sealing material 280 and a material of the base 200 so as to reduce stress applied to the joining position 270. When the base 200 includes aluminum, the flange 252 includes a nickel alloy or stainless steel.
The feed-through is soldered to the base according to the following procedure.
(1) A feed-through or base being subjected to nickel plating is coated with flux at a portion requiring good solder wetting.
(2) The feed-through is disposed on a stepped portion of a base opening.
(3) Flux is supplied into a gap caused by the stepped portion of the base between the feed-through and the base, then a solder material having an oval shape is disposed thereon.
(4) The whole base having the feed-through mounted therein is heated by a reflow furnace.
(5) When heating is finished, and cooling is completed, the residue of the flux and the like on the base are washed.
FIG. 17 shows a cross section of a soldered portion. Melted solder 300 is distributed through spreading by wetting over a narrow gap area between the feed-through 250 and the base 200. Moreover, as seen in FIG. 18 showing the solder-joined portion in an enlarged manner, a feed-through surface and a base surface, between which solder is interposed, are in a parallel relationship, and therefore most of solder is remained in the gap area, and has a planar, thinly-spread shape. Therefore, even if Sn-3Ag-0.5Cu (mass percent), of which the joining reliability level is highest among all kinds of lead-free solder, is used as solder, when a temperature cycle test is performed to the joined portion as an acceleration test, actual-use life of five years cannot be achieved, and it has been found that a crack generated in the solder-joined portion is sometimes formed into a leak path of the low-density gas.
The reason for this is that a componential material of the flange of the feed-through is Kovar™being one of iron-based materials (linear expansion coefficient: about 5 ppm/° C.) in many cases, and a componential material of the base is an aluminum-based alloy (linear expansion coefficient: about 12 ppm/° C.), that is, since a difference in linear expansion coefficient exists between the members, a solder material that joins between the materials cannot stand a thermomechanical load generated between these different materials in an early stage during actual use.
The reason why the linear expansion coefficient of Kovar™as the componential material of the flange of the feed-through needs to be significantly smaller than that of the aluminum-based alloy as the material of the base is because a seal material such as glass or ceramic is used for isolation between the flange of the feed-through and steel pins for electric signal transmission of the feed-through, and difference in linear expansion coefficient between the seal material and the material of the flange of the feed-through needs to be reduced for preventing a phenomenon that a gap is formed between both materials due to temperature change under use environment, and the low-density gas leaks through the gap.